1. Field of the Invention
This invention relates to an inverter, and more particularly, to an inverter for operating a gas discharge lamp, such as a fluorescent lamp, at a high frequency.
2. Description of the Related Art
The most common example for an inverter for converting a DC power to an AC power is a voltage resonance type inverter provided with a parallel voltage resonance circuit and a switching element. The switching element interrupts an input DC voltage at a high frequency, which is higher than an acoustic frequency, e.g., 20 to 100 kHz, and applies the thus formed AC voltage to the voltage resonance circuit. The AC voltage induced into the voltage resonance circuit is supplied to a load.
In this type of the inverter, any particular measure is not used for controlling an operating frequency, i.e., a frequency of the on/off operation of the switching element. Therefore, the operating frequency is normally fixed in the self-excited inverter, while it depends on a load to which an AC voltage is to be supplied. These inverters are disclosed in Japanese Patent Publication No. 57-45040, Japanese Patent Disclosure (Kokai) No. 61-2299, and Japanese Utility Model Disclosure (Kokai) No. 62-69396.
In the separately excited inverter, the voltage applied to the switching element depends on an operating state of the load receiving the AC voltage. In the case of the inverter followed by a load with a great load variation, such as a discharge lamp, the voltage applied to the switching element is apt to be an overvoltage. To cope with this, the switching element used must have a high breakdown voltage. This implies that the switching element for the inverter is expensive and hence the cost to manufacture the resultant inverter is increased.